Using this calculator: https://nomonsuhendar.blogspot.com/2020 ... lator.html

I plugged in the numbers for 87.7mhz but it ended up resonant on 91.9 instead. also i never got better than 1.4:1 swr and on the nanovna its a broad peak with multiple dips, not a narrow resonance like you normally get. i thought it was because i was tuning on the ground but i put it up a 30 foot mast and no difference.

I expect to have to cut and tune a bit but over 3mhz off seems like somethings wrong. and even if i get it "resonant" on 87.7 still seems like i'm gonna have bad swr either way.

Anyone ever get one of these to work right?

I've always got around 1.15 : 1 SWR, and resonance right on frequency. Are you sure that you used the right Velocity factor for the cable you're using? Are you also sure that the plastic of the tube you've housed it in is totally non-conducting? You should do the microwave oven test with your "PVC" pipe. I've found that some of them - especially grey pipes - use a metal-loaded dye to colour the plastic!

I've made them from RG58, UR76 and various other types of coax, and they've always worked well.

VF IS 0.66 which I think is standard for RG58.

First attempt used black pvc electrical conduit which I wouldn't be surprised if it was filled with carbon or something to colour it. But the second try was just coax taped to the side of the fibreglass mast with no conduit so that can't be the problem.

I tried looping a couple of turns of the coax around 3 stacked #61 ferrite cores as an extra choke, just below the coil. This made the swr even worse but more stable when touching the feedline so at least I know the coax isn't radiating now.

I tried cutting another one about 5% longer based on the theory that its tuned 5% too high. This still has the dip bang on 92.0, plus another dip around 84mhz which is what it "should" be resonant at based on the length. I'm thinking maybe its still too close the roof and either steelwork in the roof or the TV aerial mast is messing with it.

I'm gonna try making a slim jim instead and see if that works any better in the same place. Then I'll know if its the antenna thats the problem or just bad location.

It's a location problem. You need to get the antenna at least a couple of wavelengths away from everything else (about 6½m). Failing that, you can sometimes get away with 1 wavelength.

The Slim Jim is just a "completed" version of a J-pole. The only time I've seen these being really useful is in their "MSJ" variant (the "Modified" Slim Jim) which has the feedpoints moved and the length slightly extended. If done right, they work really well for vertically polarised local coverage from a very high site. Otherwise they're about as good as a properly matched vertical dipole.

I like the ⅝ and ⅞ versions of the Flowerpot - I can get useful gain out of them with a good radiation pattern.

Reason for the slim jim is its light enough to go up a fibreglass mast without making it bend. Not gonna do that with a full j pole.

You can make a J-pole even lighter than a Slim-Jim!

However - NO aerial is going to work properly unless you can get it well away from any obstructions. You wouldn't believe the number of clowns with 300 W rigs that are just heating up the asphalt on the top of the block instead of radiating a proper signal! It's almost impossible to explain to them why their aerial doesn't work. Years ago, I used to go up blocks to show them how to do it properly, and they were always astonished by the results from a correctly installed aerial!

How would one go about making a flowerpot antenna with thicker elements to run more power ? Currently there is only the rg58 design which works ok but not good for power over 50w. If the element thickness changes, the coil needs to change but I couldn’t get a good match using rg213. Only rg58 which heats up at high power.

You did change the velocity factor in the calculations? RG213 has a VF of 66, there are two flavours of RG58, the solid polyethelene stuff has a VF of 73 but the foam cored stuff has the VF of 66. Something to think about. The coil is dependent on the length of coax rather than the turns or diameter and the velocity factor changes this too.

The ideal flowerpot antenna for me would be to use only rg213 and thicker rods, like the elements of a regular dipole. Unless the centre core of rg213 is thick enough to run say 200w. If rg58 is needed there will be loses in the connection from coax to coax.